This invention relates generally to orthopedic fixation devices and, more particularly, to a variable length fixation device which is especially well-suited for fixation of fractures of the femoral neck, tibial plateau and pelvis.
Fractures of the type which commonly occur in the femoral neck, tibial plateau and pelvis require internal fixation for proper and timely healing. Such fixation is normally accomplished with nails or lag screws which hold the fractured pieces of bone together along the fracture site until healing is complete.
One known technique for fixation of fractures of the above types is the so-called Asnis Guided Screw System presently marketed by Howmedica, Inc., of N.Y., N.Y., and partially described in U.S. Pat. Nos. 4,383,527 and 4,450,835. The Asnis system utilizes one or more rigid lag screws which are installed across a fracture site over guide pins to rigidly secure the bone fragments in close proximity to one another. In treating a fracture of the femoral neck, for example, three or four screws are inserted through the lateral cortex of the femur, and extend across the fracture site and into the subchondral region of the femoral head. Each screw must be carefully selected for the exact size because they are "fixed length screws."
The rigid screws of the Asnis system may provide relatively good initial fixation of the fracture. However, during the post-operative period, resorption of bone on either side of the fracture line may occur. In this event, a fracture which has been fixed with rigid screws may no longer be firmly compressed or impacted along the fracture line. The rigid screws assume the primary load bearing function, and physiological compression which might otherwise occur at the fracture site is prevented. Alternatively, the screws may "slide" in the holes drilled in the femur as resorption occurs, and the heads of the screws will be forced outwardly away from the femur and into the surrounding soft tissues, i.e., the heads become "extruded." Under these conditions, an extended period may be required for complete and proper healing of the fracture, or the extrusion of the screws may result in loss of the fracture fixation and/or subsequent loss of the fracture reduction which may result in the fracture failing to unite which is otherwise known as a non-union.
Use of the Asnis system requires proper selection of screw size (i.e., length) to fit a particular fracture and bone size. An inventory of twenty screw sizes with lengths ranging from 35 mm to 130 mm in 5 mm increments is recommended.
Other known devices for fixation of fractures of the femoral neck are illustrated and described in U.S. Pat. Nos. 2,612,159; 2,702,543 and 4,438,762. These devices typically depend on a side plate (often referred to as a trochanteric plate in the above-referenced patents) which has a shank portion and a tubular guide sleeve which form an angle with each other of approximately 135 degrees. The shank portion is provided with a number of holes and is positioned along the lateral cortex of the femur below the area of the fracture. A plurality of bone screws extend through the holes in the shank and into the shaft of the femur to hold the plate in position. The tubular guide sleeve extends into a hole which has been drilled or reamed into the femur, and which extends along the center line of the femoral neck. A single lag screw or nail extends through the guide sleeve, across the fracture site and into the femoral head. The nail or screw is free to move telescopically within the guide sleeve so that contact between the bone fragments is maintained, notwithstanding the occurrence of resorption at the fracture site. The tubular guide sleeve of the plate, which is firmly attached to the shank of the femur, provides lateral support for the nail or screw and, at least in some cases, is designed to prevent relative rotation between the bone fragment attached to the nail or screw and the shank of the femur.
Use of a fixation device which incorporates a side plate requires the insertion of additional holes and screws into the shaft of the femur, which may already be weakened by the original injury, or by osteopathic disease. The installation and removal of these devices require relatively large incisions and a relatively complex surgical procedure. The recovery period during which the patient must be at least partially immobilized is increased, along with the probabilities of occurrence of infections and other complications.
An object of the present invention is to provide a fixation device which is simple to install and to remove, and which may be installed and removed with a minimal amount of trauma and damage to surrounding tissues.
Another object of the present invention is to provide a fixation device having a variable length which may be versatilely used in a number of fracture types and situations. A major practical advantage is one of cost since two of these screws can greatly reduce the number of device sizes which must be maintained in inventory.
Yet another object of the present invention is to provide a fixation device which avoids or minimizes other problems associated with prior art devices, while offering advantageous features to both surgeon and patient in the treatment of many types of fractures.
These and other objects of the invention are attained in a variable length fixation device for insertion into an opening formed in one or more bone fragments which comprises a barrel portion, means for securing the barrel portion to an interior surface of the opening in the bone fragments, and a fastener element. The fastener element is telescopically mounted to the barrel portion and extends into a distal one of the bone fragments for preventing lateral movements of the distal fragment relative to the barrel portion and a proximal one of the fragments. The opening in the bone fragments extends along a line which is substantially transverse to a plane of fracture between the fragments, and the telescopically mounted fastener means allows axial relative movements (i.e., movements parallel to the longitudinal axis of the fastener) of the fragments along the transverse line of the opening.
In a preferred embodiment, the means for securing the barrel portion to the interior surface of the opening comprises a plurality of locking rings formed on an outer surface of the barrel portion. The locking rings interact with a surface of the opening in the proximal bone fragment to prevent axial movements (i.e., migration into or out of the opening) of the barrel portion subsequent to insertion of the device. The fastener element preferably comprises a screw portion having a plurality of threads on a first end thereof for fastening to the distal bone fragment, and having means on a second end thereof for slidably mounting the screw portion to the barrel portion. In an especially preferred embodiment, the screw portion has a hexagonally-shaped cross-section and outer surface. In this embodiment, the barrel portion is hollow and has a hexagonally-shaped inner surface. The hexagonally-shaped outer surface of the screw portion is slidably mounted within the hollow barrel portion in an adjacent and mating relationship to the hexagonally-shaped inner surface, such that the screw portion is rotationally fixed relative to the barrel portion.
A preferred embodiment of the present invention includes means for detachably connecting a wrench to the barrel portion to allow the barrel portion to be rotated which automatically advances the screw portion into the distal fragment. This means is preferably a hexagonally-shaped recess in a proximal end of the barrel portion. In a preferred embodiment of the invention, the barrel portion and the fastener element are cannulated (i.e., hollow) and are adapted for insertion over a guide pin positioned along the transverse line of the opening. The barrel portion and fastener element are preferably permanently assembled to form a unitary fixation device.
An especially preferred embodiment of the invention is termed a Variable Length Compression (VLC.sup..TM.) bone screw assembly which comprises a barrel portion and a screw portion. The barrel portion is provided with means for securing the barrel portion to an interior surface of the opening in the proximal fragment to prevent axial movements of the barrel portion subsequent to insertion. The screw portion has a first end which is provided with means for fastening the screw portion to the distal bone fragment, and a second end which is mounted to the barrel portion. The screw portion is axially movable, but rotationally fixed, relative to the barrel portion and is preferably cannulated (i.e., hollow). In this preferred embodiment, the barrel portion is hollow and the second end of the screw portion is telescopically mounted within the barrel portion. The second end of the screw portion has a hexagonal cross-section, and an internal shape of the hollow barrel portion has a matching hexagonal cross-section such that, when the two hexagonal cross-sections are placed together in mating relationship with each other, relative rotation of the barrel portion and screw portion is prevented. The cannulated screw portion is further provided with threads on at least a portion of an internal surface to provide a means for connecting the screw portion to a compression tool. In the preferred embodiment of the invention, the means for fastening the first end of the screw portion to the distal bone fragment comprise a plurality of cancellous screw threads. Self-cutting flutes are provided on either end of the threads for easing insertion into and extraction from the distal bone fragment. The barrel portion and the screw portion are preferably permanently assembled by a swaging technique to form the complete bone screw assembly.
A preferred method of installing the bone screw assembly of the present invention includes a first step of placing a guide pin across the fracture site to define the desired location for placement of the bone screw assembly. A cannulated reamer is used to widen at least an outer portion of the hole to accommodate the barrel of the bone screw assembly. If desired or required, the remaining portion of the hole may be tapped to reduce the effort required for insertion of the screw portion into relatively dense bone tissue. After the hole has been reamed and, in some cases, tapped, the bone screw assembly is inserted into the hole, and a tool is used to turn the barrel portion, causing the barrel portion to twist the screw portion into the bone fragment. The screw portion is advanced under fluoroscopic control until the desired level of penetration has been achieved. After the required number of bone screw assemblies have been installed, the fracture may be compressed or impacted, as desired.
The design of the bone screw assembly of the present invention permits physiological compression at the fracture site during the post-operative period, notwithstanding the possible occurrence of resorption of bone tissue along the fracture. Accordingly, firm fixation is dynamically maintained throughout the period of fracture healing. Other advantages include ease of installation and removal, reduced trauma to surrounding bone and soft tissues, greater versatility resulting from the variable length feature, and reduced cost for inventory requirements.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention, when considered in conjunction with the accompanying drawings.